A printed-circuit board is a dielectric plate carrying on one or both faces a network of lines of a conductive material such as copper that are either printed on the plate or created by etching out the copper between the lines on a completely copper-plated plate. The board is drilled at the conductive lines to create holes that allow the leads of circuit elements to be soldered in place and that allow if necessary connection to be made from a line on one face of the board to a line on the opposite face. Before circuit elements are soldered to the board it is necessary to test it, both to ascertain that each of the lines is electrically continuous, and to verify that there are no unwanted connections between lines that are supposed to be electrically independent. Any error whatsoever of continuity or discontinuity makes the board usable, a reject.
The testing apparatus has a probe assembly with a multiplicity of finger-like contacts whose tips are simultaeously brought into contact with the various lines. The contacts are arrayed such that one contact engages each end of each conductive line or the end of any extension thereof. Such an arrangement is described in jointly filed and copending patent application Ser. No. 743,671 now U.S. Pat. No. 4,686,465.
To accommodate different sizes of boards the support board of the probe assembly is provided with a multiplicity of densely spaced depressible supports. A tight grid of such supports is provided, with the supports independently connected to an electronic test and analyzing device, and the grid is at least as large as the largest board to be tested. Thus when the board to be tested and the array of supports are juxtaposed, there will be at least one support generally aligned with the end of every line or segment thereof as well as with any bores in the board. The contacts are rods supported on those supports which lie generally in the right position relative to the locations to be contacted. Only those contacts needed for a specific test are mounted on supports, and a mask is provided to orient the forward ends of the contact rods appropriately for the specific board being tested. The supports allow the contact rods to be depressed back into the probe assembly so that when the board is pressed back or the assembly is pressed up against the board the contacts will all bear with a certain standard pressure against the points being tested, thereby making the resistance at the interface between the front end of the contact and the line being tested uniform for all the contacts.
For best contact with a standard printed-circuit conductor line it is standard to use contact rods whose forward contact ends are somewhat pointed. When they are to be used to test a grommet, socket, or similar bushing fitted in the board it is standard as described in U.S. Pat. No. 3,676,776 to form the contact ends of the rods as balls so that they will naturally fit and center themselves within the invariably circular holes and collars on the boards. This patent also describes an arrangement wherein the contact rod is cylindrical with a squared-off front end which carries a part-spherical removable tip. It is further possible to form the tip with a rearwardly projecting end pin that can fit in the front end of the rod. Such an arrangement allows the tips to be changed for different applications, nonetheless it requires that a complete set of tips be maintained on the contacts at all times, and the necessary construction is often expensive or requires relatively thick contact rods to be used.